A linear sensor which has a plurality of rows, for example, three rows formed on a chip is known. The linear sensor is used in an image information input unit such as a color image scanner or a digital color copying machine. In this three-row linear sensor, the output signals from the rows should be output from one place because the signal processing system can be simplified as compared with the case in which the output signals from the rows are output from three places.
FIG. 1 shows a conventional linear sensor. The conventional linear sensor includes three rows 11 to 13 which are arranged in the vertical direction (V-direction). Each of the sensor rows have a predetermined number of pixels one-dimensionally arranged in the horizontal direction (H-direction). Analog shift registers (hereinafter, called the HCCDs) 21 to 23 are formed of CCDs (Charge Coupled Devices) for transferring signal charges in the H-direction. Shift gates 31 to 33 are formed for shifting the signal charges accumulated in the pixels of the sensor rows 11 to 13 to the HCCDs 21 to 23.
The signal charges transferred by the HCCDs 21 to 23 are converted (or reset) into a voltage by, for example, charge/voltage converter units 51 to 53 of a floating diffusion amplifier provided at the output of the HCCDs 21 to 23. Switches SW1 to SW3 are provided at the output sides of the charge/voltage converter units 51 to 53. These switches SW1 to SW3 are controlled to be sequentially changed in their positions, so that the signals are output from the rows in order.
FIG. 2 shows a timing chart of two different-phase horizontal clocks .PHI.1,.PHI.2, read-out pulses .PHI.rog1 to .PHI.rog3 for shift gates 31 to 33 and an output signal Vout, which are indicated in FIG. 1. The period tint is the signal charge accumulating time for each row.
As described above, in the conventional three-row linear sensor in which the output signals from the rows are output from one place, the switches SW1 to SW3 must be provided at the output sides of the HCCDs 21 to 23 to switch the output signals from the rows. Therefore, when the output signals are to be continuously read, it is necessary to shift the sampling times for the optical signals on the rows. The image information on the three rows cannot be two-dimensionally sampled at substantially the same time.
FIG. 3 shows another conventional linear sensor. As shown this figure, several to several tens of pixels of each row are shaded to be optical black (OPB). The outputs from the OPB pixels are used to clamp for the black level correction.
FIG. 4 shows a timing chart of two different-phase horizontal clocks .PHI.1, .PHI.2, read-out pulses .PHI.rog1 to .PHI.rog3 for shift gates 31 to 33 and an output signal Vout, which are indicated in FIG. 3. The period tint is the signal charge accumulating time for each row.
When the number of pixels of each row is large, or when the transfer speed is slow, however, the HCCDs 21 to 23 cause undesirable dark current components. The signals from the pixels more separated from the OPB pixels in the horizontal direction have the lower S/N ratios due to the dark current components.